Maybe a dumb question but I have to ask this about props and gears.

[John800]

So, looking through specs for different outboards I see some slightly different models of the same hp with different gear ratios. My question is, is there an advantage to doing that with gears vs prop pitch? Is a certain pitch prop of the same design more efficient?

[nitro_rat]

lower pitch numbers are more efficient. Somewhere after 30 props kinda start pushing more sideways like a paddle than forward. Prop pitch is a measurement of linear travel per revolution. A 24 pitch prop theoretically travels 24" for each revolution. Slip kinda mucks all this up in practice but that's why different gear ratios are available.

[HStream1]

Heres a response I posted a couple years back when this same question popped up.

There are 4 lower merc gear ratios.

2:1, 1.87:1, 1.75:1, 1.62:1.

It works like this:
2:1 is two engine rotations to one Prop rotation

1.87:1 is 1 7/8 engine rotation to one prop rotation

1.75:1 is 1 3/4 engine rotation to one prop rotation

1.62:1 is 1 9/16 engine rotation to one prop rotation

In theory heres the difference, and Ill use 5000 RPM and a 26 pitch prop as a base line for the example.

The formula to calculate Theoretical speed is:
RPM X Pitch =
Ratio X 1056 =

2:1
5000 x 26 = 130000 = 61.5 MPH @ 0% slip
2 x 1056 = 2112

1.87:1
5000 x 26 = 130000 = 65.83 MPH @ 0% slip
1.87 x 1056 = 1974.72

1.75:1
5000 x 26 = 130000 = 70.34 MPH @ 0% slip
1.75 x 1056 = 1848

1.62:1
5000 x 26 = 130000 = 75.99 MPH @ 0% slip
1.62 x 1056 = 1710.72

So as you can see in theory by using 5000 RPM and 26 Pitch as a Base line the different ratios show around a 5 MPH difference. Now many variables come into play here, Boats weight, Hull design, Prop efficiency (% percentage of Slip), Can the motor carry a taller pitch pitch prop all the way through its maximum RPM range?

Now lets look at it a different way changing only prop pitch by 2 (ex 24 P vs 26 P) for this example ill use 1.87:1 as the base because as shown in the theory above the different ratios equate to around 5 MPH.

5000 x 26 = 130000 =65.83 MPH @ 0% slip
1.87 x 1056 = 1974.72

5000 x 24 = 120000 =60.76 MPH @ 0% slip
1.87 x 1056 = 1974.72

As you can see by merely changing the prop by 2 pitch sizes the MPH difference is almost the same @ 5 MPH difference.

Again many variables come into play and the above is based on Theory not real world results.

Also the tighter gear ratios while providing a faster top end speed will also show a more sluggish hole shot.

To calculate Slip take Theoretical Speed minus Actual speed = % slip.